Speed and Space Management
Speed and space management is the sixth domain of the Louisiana CDL General Knowledge exam. It tests whether you choose a safe speed for conditions and keep enough space around the vehicle to stop, steer, or escape.
Speed
The posted limit is a maximum for ideal conditions, not a target. Reduce speed for rain, fog, ice, curves, construction, heavy traffic, and a loaded or high-center-of-gravity vehicle. Total stopping distance includes perception distance, reaction distance, brake lag, and braking distance-so a heavier vehicle at the same speed needs more room.
Following distance
For speeds under 40 mph, allow one second for each 10 feet of vehicle length. Above 40 mph, add one second. A 50-foot vehicle needs 5 seconds below 40 mph and 6 seconds above; a 60-foot combination needs 6 and 7 seconds.
Space around
Avoid tailgaters by increasing your own following distance and, if possible, changing lanes. Leave room beside the vehicle so you are not trapped, and plan an escape route at intersections and in construction zones.
Worked example
A 60-foot combination travels at 55 mph. The minimum following distance is 7 seconds. At 55 mph, a vehicle covers about 81 feet per second, so 7 seconds is roughly 565 feet-more than a football field. The reasoning: the length formula accounts for the vehicle's size, and the extra second above 40 mph accounts for longer stopping distances.
Traps
Using passenger-car following distances, counting seconds too quickly, and ignoring trailer length or road conditions are common errors.